1. Field of the Invention
The present invention relates to a semiconductor laser, more specifically, to a self-oscillation laser (pulsation laser), and a method of growing crystals, such as aluminum mixed crystals, on a substrate having a stepped major surface.
2. Description of the Prior Art
Semiconductor lasers have been widely employed as light sources for the optical read/write operation with recording media like optical disks and magnetooptic disks. In recording information on or reading information from an optical recording medium, such as an optical disk or a magnetooptic disk, with a semiconductor laser, an inevitable small quantity of reflected light is returned from the optical recording medium or the optical system which produces reflected light noise due to interference with the laser beam to cause significant problems.
Such problems attributable to reflected light noise may be solved by employing a multimode semiconductor laser or a pulsation semiconductor laser, or by the fast modulation of the semiconductor laser.
As is generally known, a gain guide semiconductor laser incorporating a gain waveguide is capable of multimode oscillation. However, the thresho)d current I.sub.th of the gain guide semiconductor laser is very high, and such a very high threshold current I.sub.th is an impediment to increasing the output of the semiconductor laser for satisfactory operation in writing signal in and reading recorded signal from the magnetooptic recording medium or an optical disk by using the semiconductor laser as a light source. The fast modulation of the semiconductor laser requires a circuit of a complicated configuration.
Accordingly, the use of a self-pulsation (self-oscillation) laser is desirable to reduce the reflected light noise. The pulsation of the self-pulsation laser can be achieved by a first means that employs an intermediate construction between an index guide and a gain guide, a second means that employs a saturable (supersaturation) absorber or a third means that increases light expansion beyond current expansion. The first and second means are effective, in practical application, in controlling the pulsation, and increases the threshold current I.sub.th.
To solve such problems, the applicant of the present patent application proposed a method of forming a buried heterostructure semiconductor laser in Japanese Patent Laid-open (Kokai) No. 61-183987. This method utilizes the dependence of epitaxial growth rate on the orientation of the crystal face in forming a semiconductor laser through a single, continuous process. This method forms semiconductor layers continuously and sequentially over the surface of a semiconductor substrate provided with a strip mesa by epitaxial growth to form a buried heterostructure semiconductor laser having a comparatively low threshold current I.sub.th.
The applicant of the present patent application further proposed semiconductor lasers formed by utilizing the dependence of epitaxial growth rate on the orientation of the crystal face in Japanese Patent Laid-open (Kokai) Nos. 62-217829 and 63-330136. These semiconductor lasers incorporate improvements for simplifying the manufacturing process and further stabilizing the characteristics. FIG. 16 shows one of these previously proposed semiconductor lasers. As shown in FIG. 16, the semiconductor laser is formed by sequentially forming a first conduction type, for example, n-type, AlGaAs cladding layer 3, a low-impurity or undoped GaAs active layer 4, a first, second conduction type, for example p-type, AlGaAs cladding layer 5, a first conduction type, for example n-type, AlGaAs current blocking cladding layer 6, a second, second conduction type, for example, p-type, AlGaAs cladding layer 7 and a second conduction type cap layer 8 on the major surface 1a of a substrate 1, such as a GaAs compound semiconductor substrate, having the crystal face (100) and provided with a strip mesa 2 extending along the &lt;011&gt; crystallographic axis by a metal organic chemical vapor deposition (MOCVD) process.
First electrode 9 and a second electrode 10 are formed respectively over the cap layer 8 and the backside of the substrate 1 in ohmic contact. The n-type cladding layer 3, the first p-type cladding layer 5, the second p-type cladding layer 7 and the n-type current blocking layer 6 are formed of materials having a large bandgap, that is, a small refractive index, as compared with that of a material forming the active layer 4.
In depositing the materials in crystals by the epitaxial growth method, portions of the lower layers formed over the strip mesa 2 are superposed in a laminated structure having a triangular cross section, because, once the (111)B crystal face is formed by epitaxial growth on the strip mesa 2, the epitaxial growth rate of crystals on the (111)B crystal face drops below a few tenth of the epitaxial growth rate of crystals on other crystal face, for example the (100) crystal face, the epitaxial growth of crystals on the (111)B crystal face barely continues and the (111)B crystal face forms a fault. Thus, the relation with the orientation of the crystal face of the strip mesa 2 of the substrate 1 is specified, the shape and size of the strip mesa 2 are determined selectively and the respective thicknesses of the layers 3 to 6 are determined selectively to form a strip of the active layer 4 between the cladding layers 3 and 5 formed on the strip mesa 2 between inclined side surfaces 11 of the (111)B crystal face inclined at about 55.degree. so that the strip of the active layer 4 is separated from other portion of the active layer 4, and to form the current blocking layer 6 contiguously with the side edges of the strip of the active layer 4, hence, the inclined side surfaces 11, in mesa grooves 12.
Since the strip of the active layer 4 formed on the strip mesa 2 is surrounded by the current blocking layer 6, the semiconductor laser having a low threshold current I.sub.th can be fabricated through a single cycle of continuous epitaxial growth process.
The semiconductor laser thus formed by the aluminum mixed crystals grown by epitaxial growth through the MOCVD process, however, has problems that depressions or irregularities are formed at some positions in a portion of the electrode 9 corresponding to the strip mesa 2 and that the laser beam does not oscillate, the oscillating efficiency is low and the characteristics is unstable when such depressions or irregularities are formed in the electrode 9.
It is known that the epitaxial growth rate increases sharply with increase in the aluminum content of the aluminum mixed crystals. The inventor of the present invention have found through studies that the deterioration of the characteristics of the semiconductor laser is more or less attributable to the epitaxial growth rate. For example, an aluminum oxide is deposited across the strip of the epitaxial semiconductor layer formed separate from other portion of the same epitaxial semiconductor layer on the strip mesa 2 forming depressions or irregularities in the electrode 9 and impeding current concentration on the active layer 4 or intercepting current. Such depressions or irregularities make the semiconductor laser unacceptable and cause problems such as the deterioration or fluctuation of the characteristics.